[->] (R) – (S) node[anchor=south] \(\tau \) ;
[->] (S) –(B) node[anchor=south]\(a\) ;
[->](S)–(C) node[anchor=north]\(b\) ;
[->](B) –(C) node[anchor=west]\(c\) ;


where the construction of \(\tau \) , which is surjective, is given on page 79 of [1]} There is a typo in this reference, the inverse sign on two of the generators (\(T_2\)  and \(T_4\) ) is missing., and \(a\) , \(b\) , and \(c\)  are naturally induced. The two isomorphisms follow from (REF ) and (REF ). Since all the morphisms are surjective and \(b=c\circ a\) , see page 13 of [2]}, it follows that \(\text{Ker}(b\circ \tau ) \supset \text{Ker}(a\circ \tau )\)  and \( \text{Ker}(b\circ \tau )/\text{Ker}(a\circ \tau ) \cong \text{Ker}(c)\) . This gives the short exact sequence
\(1 \rightarrow \text{Ker}(a\circ \tau ) \rightarrow \text{Ker}(b \circ \tau ) \rightarrow \text{Ker}(c) \rightarrow 1.\) 
